The present invention relates to a magnetic resonance (MR) imaging system using a superconducting magnet as a source for generating a static magnetic field, and, in particularly, to a magnetic field generating system which reduces the influence of an eddy current at the time a gradient field is applied in a pulse shape.
In MR imaging systems, a magnetic resonance phenomenon occurs when a uniform static magnetic field as well as a pulsed gradient field and a pulsed high frequency field are applied to a living body under examination. Further, in the MR imaging systems, a magnetic resonance signal produced from within the body is detected on the basis of the magnetic resonance phenomenon and is subjected to a process including an image reconstruction. As a result of this process, the MR imaging systems provide image data representing a spatial distribution of nucleus spins of a particular atomic nuclide in the body under examination.
The MR imaging systems require that a static magnetic field with high intensity, uniformity and stableness be generated. In particularly, it is necessary to provide a high S/N (signal-to-noise ratio) and a high resolution in order to obtain more delicate and complex magnetic resonance data, which may be used for imaging a chemical shift or a number of nuclide or nuclear species. Then, a static magnetic field with high intensity, uniformity and stableness needs to be generated. A superconducting magnet is suitable as a source to generate a static magnetic field that satisfies the above conditions.
The basic structure of a superconducting magnet used as a static magnetic field generating source in MR imaging systems will now be explained. A superconducting coil made of a superconducting wire material is wound around a bobbin made of metal that is non-magnetic even at a low temperature, and the overall coil assembly is dipped in a liquid helium serving as a cooling medium. The outside of the liquid helium, up to the outermost layer that communicates with the ambient temperature, are covered by a plurality of heat shielding plates made of high thermal conductivity, with layers of liquid nitrogen sandwiched therebetween. Of the multilayered structure, those layers which do not contain the liquid helium or the liquid nitrogen are made vacuous. Such a low temperature container is typically called a cryostat. The heat shielding plates are made of a metal with a high thermal conductivity and a non-magnetism at a low temperature, typically of aluminum.
In the MR imaging systems, gradient coils are provided inside the static magnetic field generating superconducting magnet. Consequently, the flux generated by the gradient pulse, or the gradient field generated in a pulse form, from the gradient coils interlinks with the metal member (e.g., the heat shielding plates) constituting the cryostat and as change in the interlinked flux generates an eddy current. The flux generated by the eddy current distorts the pulse shape of the gradient pulse.
This distortion process is illustrated in FIGS. 1A through 1D. FIG. 1A illustrates a current flowing across the gradient coil and FIG. 1B illustrates the waveform of a gradient pulse H.sub.1. A change in flux caused at the rising or falling of the gradient pulse H.sub.1 causes an eddy current to flow at the metal member of the superconducting magnet, and this eddy current causes a magnetic field H.sub.2 as shown in FIG. 1C to be generated from the metal member as a consequence. As illustrated in FIG. 1C, the eddy-current-originated field H.sub.2 is generated at the rising and falling edge portions of the gradient pulse H.sub.1. The attenuation constant of the field H.sub.2 caused by the eddy current is determined by the material and the shape of the metal member. As this field H.sub.2 is superimposed on the original gradient pulse H.sub.1, the resultant gradient pulse H.sub.1 would have a waveform which becomes blunted at its rising and falling edge portions, as shown due to the pulse H.sub.1 ' of FIG. 1D. This deformation of the gradient pulse is one cause to deteriorate the image quality and should therefore be dealt with.
To suppress the magnetic field caused by the eddy current, a metal of low electric conductivity, such as stainless steel, may be used a the metal member for the heat shielding plates, etc. so as to make it difficult for the eddy current to flow therethrough, or the space between the gradient coil and the superconducting magnet (accommodated in the cryostat) used for generating the static magnetic field may be set large.
However, because of its low thermal conductivity, the stainless steel increases the loss by the liquid helium and liquid nitrogen so that the former method of using the stainless steel is not suitable. The latter method requires that the image pick-up space in which the body under examination comes should be made smaller to reduce the diameter of the gradient coil or the diameter of the superconducting magnet should be increased, thus enlarging the imaging apparatus. Accordingly, this latter method is not suitable, either.
As should be clear from the above, according to the conventional MR imaging systems using a superconducting magnet, it is difficult to remove the influence of the magnetic field caused by the eddy current flowing through the metal member of the superconducting magnet by the gradient pulse.